This invention relates to a process for stabilizing the fiber orientation and fiber distribution in webs of textile-length fibers which are intended to be eventually processed into non-woven fabrics. More particularly it relates to a process for bonding the fibers of a fibrous web, using short binder fibers, so that they maintain their general positional relationships through subsequent operations of drafting, printing, saturating, drying, winding into roll form, and the like.
Nonwoven fabrics are a recognized article of commerce widely used for the formation of disposable or semi-disposable articles such as sheets, pillowcases, hospital gowns and drapes, wiping cloths, dusters, and for a wide variety of other purposes. The most common type of nonwoven fabric is that which is formed by saturating or impregnating with a polymeric dispersion or latex an unspun, unwoven, intermingled array of textile-length fibers delivered from a card, garnett, or air-lay machine. By far the most common web-forming device is a so-called card or carding machine, which comprises a cylinder, three to four feet in diameter, and of any desired width, covered with a multiplicity of fine bent wires called teeth or card clothing. A lap or roll of fibers is fed by means of a so-called licker-in roll to this cylinder, which picks up and carries a fleece or veil of fibers. The fibers are also worked upon by auxiliary rolls or flat strips covered with wire teeth and set in close proximity to the circumference of the cylinder. The fibrous web is removed from the cylinder by a doffing device, which is conventionally a secondary wire-covered cylinder tangential to the main cylinder and revolving in the opposite direction. From the doffer, the web is removed and forwarded for further processing by a vibrating comb or similar device.
The conventional card or carding machine was primarily devised to exert a two-fold effect. First, it cleaned an array of fibers such as raw cotton or wool, removing considerable dirt, other foreign matter, and short fibers. Second, it tended to parallelize and rearrange the fibers, which was then a desirable consequence since the fibrous card web was customarily further drafted and spun into yarn in a multi-stage process.
With the advent of the nonwoven fabric industry, however, many cards were converted from their original function of preparing a sliver of parallel fibers to delivering a full-width web or a plurality of such webs to a conveyor belt, for subsequent bonding to form a nonwoven fabric. One of the previous advantages of carding, parallelization of the fibers, became a distinct disadvantage in the preparation of nonwoven fabrics. Spun yarns have strength only in the lengthwise direction, where the strength is needed for conventional weaving purposes, whereas the majority of nonwoven fabrics must have at least some minimal tensile strength in the transverse or cross-web direction. The problem is particularly acute in the case of nonwoven fabrics prepared from man-made fibers, which, although they may be crimped, are generally straighter and much more readily oriented than natural fibers such as cotton.
A further complication is that the conventional processing of bonded nonwoven fabrics consists of a set of stages -- conveying, saturating, drying, winding, etc. -- all of which impose a further drafting and parallelizing effect on the fibrous web, since the web is being stretched under tension in each of these operations. If a web is carefully removed from the doffer of a card and then is bonded to form a nonwoven fabric with careful handling to avoid drafting or distortion, the longitudinal (machine direction or M.D.) tensile strength may be only 3 or 4 times the lateral (cross-direction or C.D.) tensile strength. In conventional multi-stage bonding and drying operations, however, wherein the web is pulled under tension in each processing stage, tensile strength ratios of 10 or 20 to 1 are found, M.D. to C.D.
Various expedients are resorted to for improving what will hereinafter be referred to simply as the strength ratio, it being understood that this refers to the ratio of strength in the machine or longitudinal direction to the cross or lateral direction. One expedient is to disperse the fibers in more or less random orientation into an air stream, from which they are collected on a perforated rotating drum by suction. Such devices are expensive, and clumping and poor fiber dispersion appear at the speeds of 30 to 50 yards per minute at which it is economical to process nonwoven fabrics.
Another common method of improving the strength ratio is by means of a cross-laying device, whereby a full-width web of oriented fibers is mechanically pleated back and forth across a conveyor belt to build up a composite batt in which the average angular displacement of the fibers is alternated. Such devices again are slow, cumbersome, and are suitable only for batts of substantial thickness where fold marks and overlap ridges are not objectionable.
Other auxiliary devices have been proposed to randomize carded webs, such as that set forth in U.S. Pat. No. 3,538,552 to J. L. Foley, of common assignee.
Still other devices for creating webs in which the fibers lie in more or less random orientation are described in my copending application Ser. No. 159,229, filed July 2, 1971, and U.S. Pat. No. 3,727,270, issued Apr. 17, 1973. In these applications, textile-length fibers are accelerated and drafted through aspirators, diffused and decelerated in a plenum chamber, and eventually collected in the form of randomly-oriented webs.
However, as set forth above, devices which randomize fibrous webs from the various types of web-forming devices are generally ineffective in maintaining a favorable M.D. -- C.D. strength ratio during the subsequent operations of saturating and drying, which are essential steps in the preparation of most bonded nonwoven fabrics. The fibrous webs are drawn or drafted through the bonding and drying steps, and since the fibers are only casually engaged by frictional forces, they become gradually more and more oriented in the machine direction -- that is, lengthwise of the web. In this way, a random web which is delivered from the web former with approximately the same crosswise as lengthwise strength may show a M.D. tensile strength which is 4 or 5 times the C.D. strength after complete processing. In products where the C.D. strength is a controlling or essential factor, this means that the nonwoven fabrics must be made heavier than need be, to meet the minimum crosswise strength requirement, which is a wasteful expedient.
This invention is directed toward the use of short thermoplastic, thermoretractile fibers as a prebond for increasing the strength ratio of a web preliminary to the subsequent operations of saturating, drying and the like. It is known to subject fibrous webs comprising a mixture of thermoplastic and non-thermoplastic fibers to heat, in order to form a nonwoven web, as taught by Reed in U.S. Pat. No. 2,277,049 and by Harrington, Jr. et al in U.S. Pat. No. 3,229,008. However, the teaching in both such instances relates to carded or garnetted webs in which both the thermosensitive and non-thermosensitive fibers are of textile-length, that is, capable of being formed into webs by dry-lay mechanical processing. These thermosensitive fibers have sufficient length, therefore, to become reticulately entwined with the similarly long non-thermosensitive fibers. Since these fibers are entwined, or interlaced, with adjoining fibers, then, upon heating in the absence of pressures, when these thermosensitive fibers go through the process of retraction or fiber shrink, they pull the non-thermosensitive into an area contraction. This shrinkage of the textile-length thermosensitive fibers sets up a shrinkage tendency in the web as a whole, since the retracting fibers, spanning over, and interlaced with, many non-thermosensitive fibers draw these fibers together as retraction sets in. As a practical matter, therefore, the bonding processes in U.S. Pat. Nos. 2,277,049 and 3,229,008 are always carried out under pressure if undesirable shrinkage is to be avoided. Although the use of a pressure nip has eliminated this shrink, it has done so under the penalty of compressing the fabric, destroying its loft, and introducing processing difficulties.
Accordingly, it is an object of this invention to provide a process for making a web of mixed thermosensitive and non-thermosensitive fibers which can be heated at zero pressure to activate the thermosensitive (binder) fibers without significant shrinkage of the web, while enhancing the homogeniety of the distribution of those binder fibers within the web.
It is another object of the present invention to provide a process that stabilizes the fibers in an air-laid randomly oriented web against the gross distortion and reorientation which they normally are subject to in subsequent bonding and drying operations.
It is a further object of the invention to provide a method of making a bonded air-laid web wherein a substantial number of the fibers in the web are bonded only at their crossover points, whereby the web remains flexible, and substantially uncondensed.
Still another object of the invention is to provide a process that limits the maximum size of the bead resulting from the melting of a thermoplastic fiber.